Thursday, June 21, 2007

The bacterial cell wall component LPS (lipopolysaccharide) binds to Toll-like receptor 4 (TLR4) on macrophages and stimulates them to produce the strong pro-inflammatory cytokine TNF (tumor necrosis factor). To identify potential signaling proteins, Kang and colleagues used the intracellular domain of TLR4 in a two-hybrid screen of binding proteins. They isolated 4-1BBL (CD137L), a transmembrane protein previously best known as a T cell costimulator, and the well-known TLR signaling protein MyD88. 4-1BBL binding was not abrogated by a P712H substitution in the TLR4 TIR (Toll-IL-R) motif that is required for MyD88 binding, indicating that they bind to different parts of TLR4.

LPS induced less TNF production at 24 h from 4-1BBL “knockout” (4-1BBL KO) macrophages than from wild-type (WT) macrophages, though equivalent amounts of IL-1beta were produced. Moreover, kinetic analysis demonstrated that TNF production was equivalent soon after stimulation but ceased after about 6 hours in 4-1BBL-KO macrophages while continuing in WT macrophages (Figure, panel b), accounting for the difference at 24 h. Although 4-1BBL was cloned as the ligand for 4-1BB, which is also expressed by macrophages, it does not contribute to this response (Figure, panel c).

LPS induces a gradual appearance of 4-1BBL on the cell surface, with marked accumulation after 2 hours, which accounts for the delayed influence on TNF production. Indeed, surface 4-1BBL alone at high levels of expression, or cross-linked at lower levels of expression, is sufficient induce TNF expression. Finally, in the ultimate test of relevance, they showed that 4-1BBL-KO mice survived a dose of LPS that killed all WT mice. Their discovery of 4-1BBL's role in sustained TNF production provides a new target for therapeutic intervention in the development of inflammatory diseases.

Mission

Think for yourself! BioMed Notes is for news and discussion of life science research.

You can post a comment.

You can email a story by clicking on the envelope icon beneath the introduction. Include the data supporting the key observation (figure, table, or text).

You can start a discussion by submitting a brief introduction to a paper. Click here: Submit Story to start an email to the editors. We will also consider posting good ideas that haven't been published.